Similar words to "Underline" and their differences

emphasize, stress and underline are generally interchangeable- but stress and underline have other meanings.
stress can mean a physical pressure, e.g.
"the distribution of stress is uniform across the bar."
stress can also be an emotional state, where someone is mentally under pressure. e.g.
"my friend was under a lot of stress."
underline can be the action of drawing a line under a word, or phrase in ORDER to give emphasis. this is why underline is also used as a synonym of emphasize and stress. e.g.
"certain phrases have been underlined."
hope this helps! if you have any questions, feel free to ask.

So, "apt" means capable or good enough to do something. If you say, "They are apt enough to have fights", then you are saying that they have the ability to have fights. This doesn't mean that they have an unhealthy mind, it just means that they can do something unhealthy. It would be better to say, "This is because those people look to have fights with others" because they are searching to make trouble.

This is pretty good, but some words are unneccessary and I would break it up into multiple sentences. For example:
His father tried to persuade his son not to leave home, underlining how fortunate he was to live a carefree life where poverty was a distant reality. He added what happened to the boy's elder brother, who lost his life in battle after deciding to join the armed forces despite his father's complaints.
("Complaints" works at the end, but "misgivings" would probably be a better word.)

"His father tried to persuade his son not to leave, saying how fortunate he was to live such a carefree life where poverty was a distant reality. Like what happened to his elder brother, who, despite his father's continual complaints, decided to join the armed forces and lost his life in battle."
I'm not 100% sure this is what you mean. The sentence is more unclear than incorrect. It's unclear because it's a very LONG sentence. The longer the sentence the harder it is to understand.
The meaning of the word "underlining" is also unclear in this sentence. It seemed to imply that you (the person writing the sentence) thinks that the son leaving morally shows how priverlaged the son is?
Or it could be interpreted as the father telling his son he has such a good life?

After the storm and when everything returned to normal, Robinson forgot all about his prayers in which he asked God to spare his life; especially the promise that he would never again put to sea (never set sail again). But, as often happens when things look calmer, humans seem to forget the fear they felt in dangerous (life-threatening) situations, and they return to what they desired. In the same way, Robinson 's desire to return to the sea appeared again. (Robinson's hunger for the sea reappeared)
—My suggestions are in ( )

YouTuber just refers to anyone who makes YouTube videos, regardless of topic or content, but a vlogger is specifically someone who makes videos about their daily life/experiences, or just their thoughts about things.
So someone who plays games and posts those videos on YouTube isn't a vlogger, but they are a YouTuber. You can be a vlogger without being a YouTuber if you don't upload your videos onto YouTube.